Dehydrohalogenation of organic compounds



United States Patent DEHYDROHALOGENATION OF ORGANIC COMPOUNDS George McCoy, Philadelphia, Charles E. Inman, Roslyn, and Glendon D. Kyker, Glenside, Pa., assignors to Pennsalt Chemicals Corporation, a corporation of Pennsylvania 4 Claims. (Cl. 260-650) This invention relates to the dehydrohalogenation of l organic compounds by pyrolysis and more particularly to improvements in the dehydrohalogenation of cyclic halogenated compounds by pyrolysis to produce compounds having conjugated systems and aromaticity.

The terms aromaticity and aromatic character as used herein, both in the specification and claims, are intended .to indicate those compounds of peculiarly diminished unsaturation and pronounced tendency to the formation and preservation of type such as-benzene, pyridene, pyrrole, thiophene, pyrazole, furane, etc, This application is a division of our application, Serial No. 269,323, filed January 31, 1952, now US. Patent No. 2,914,573.

It has heretofore been known that compounds of aromatic character having conjugated systems such as benzene and thiophene can be formed through the dehydrohalogenation of certain chlorinated cyclic compounds through the application of heat. F. E. Matthews, for example, in his article The a and B Modifications of Benzene Hexachloride, Journal of the Chemical Society of London, vol. 59, Trans. 168-169 (1891), indicates that trichlorobenzene can be formed directly from benzene'hexachloride by heating benzene hexachloride to sufliciently high temperatures to split out HCl. However, when attempts were made to convert chlorinated cyclic compounds into compounds of aromatic character having conjugated systems, as for example, the conversion of benzene hexachloride to trichlorobenzene, or

the conversion of tetrachlorothiolane and hexachlorothiolane, respectively, to dichlorothiophene and tetrachlorothiophene through the application of heat alone it .:.*as found that relatively high temperatures were necessary. 1 Thus, for the pyrolytic decomposition of benzene hexachloride to trichlorobenzene a temperature in excess of 500 C. was necessary before any appreciable i dehydrohalogenation occurred.

l The high temperatures necessary for carrying out these reactions by heat -alone are unsatisfactory for several reasons. At elevated temperatures care must be taken in the selection of materials of construction for the apparatus employed since there is a tendency for most metals to be attacked by the reactants at elevated temperatures, the degree of attack increasing with increase in temperature. This is particularly true where dehydrohalogenation and chlorination are carried out simultaneously as described in U.S. Patent No. 2,778,860. Also, at temperatures much in excess of 300 C., there is a distinct tendency for the carbon to carbon linkage of many cyclic organic materials to be broken resulting in the production of undesirable gummy materials and free carbon which must later be removed. A, further disadvantage with respect to the use of high temperatures is the increased cost of operation due to the expense of heating the equipment and the reactants. Another objection to the use of heat alone for carrying out the reaction is the relatively slow rate at which dehydrohalogen'ation occurs particularly if attempts are made to carry out the reactions at sufficiently low temperatures to avoid at least in part the above mentioned difficulties.

When the metal and metal halides which are customarily used as catalysts in the dehydrohalogenation of chlorinated aliphatic compounds are employed, aslight reduction in temperature is obtained. However, the temperature necessary to obtain a suitable rate of dehydrohalogenation is still unsatisfactorily high, being in most instances above 400 C.

We have now discovered that reactions of the type described can be substantially accelerated and carried out at substantially lower temperatures it carried out in the presence of activated carbon. In most instances, through the use of activated carbon as a catalyst, the dehydrohalogenation can be carried out at temperatures as low as 190 C. Also, substantially all of the halogenated cyclicorganic is converted into the desired prod- This is surprising when it is considered that activated carbon, when employed as a catalyst in the dehydrohalogenation of chlorinated aliphatic compounds,

is considerably inferior to metal and metal halides such I TABLE First 15 minutes reflux of slurry of 100 grams benzene hexachloride in 100 grams of trichlorobenzene and catalyst and materials such as kaolin and Ratio of Reflux Catalyst] g. HCl Tempera- BHC ture, C.

Activated Carbon:

Columbia SXW 1:15 15. 5 217 Columbia G 1:15 15.3 217 Columbia L- 1 :15 20. 6 210 Columbia SXc 1:15 15. 0 217 Columbia CXA 1:5 30.8 210 Columbia CXA 1:10 251 212 Columbia CXA 1:15 20. 9 214 Columbia CXA 1:20 17. 5 217 or 211 1:15 24.4 211 Norit SG II Extra..- 1:15 29. 0 210 Darco G- 1:15 24,8 212 Darco 5-51- 1:15 15. 3 214 Nuchar C-N 1:15 14. 1 226 Chlichar Activate 1:15 142 217 Carbon (not activated): 1 V Cliffchar Unactlvated 1:15 0.2 232 Tenn. Prod. & Chem. 00.-.- 1:15 0.1 232 Consolidated Chem. Ind 1:15 0.6 228 FeCla 1:15 0.0 232 1 :15 0. 0 232 1:15 0.0 230 1:15 0.0 232 1 :15 0. 0 228 Glass beads.-. 1:15 0.0 232 steam).

The temperature at which dehydrochlorination can be carried out when usingactivated carbon as a catalyst is sufficiently low. to permit the use of glass or glass-lined reaction vessels without fear of the glass softening at the temperature employed. Since glass is substantially unattacked by HCl or chlorine, the problem of corrosion of the apparatus employed is essentially avoided. The temperatures employed are also sufficiently low to permit the dehydrohalogenation to be carried out in jaliquid medium. This is frequently desirable, particularlyin batch processes Where the reaction can be carried out in a liquid medium which is the same as the product being .obtained. The use of activated carbon as a catalyst, howeven-isnotlim- 'ited to liquid phase reactions, since it is found that activated carbon also considerably enhances the reaction-rate and reduces the reaction temperature where the materials being subjected to dehyd'rohalogenation are heated without any dilution, or where the reaction is carried out in the vapor phase.

The general concept of our invention is particularly useful in the production of trichlorobenzene from benzene hexachloride. The invention, however, is not limited to the production of these materials alone, since our inyention also has considerable utility in .the production of other cyclic chlorinated compounds of aromatic character having conjugated systems from halogenated cyclic compounds.

One of the primary reasons for preparingbenzene hexachloride is the large demand for the gamma isomer, which has come to have increasing application as an insecticide.

However, there is, at present, no practical commercial way of chlorinating benzene so as to obtain substantially pure gamma benzene hexachloride as the resulting prodnot. In the present commercial processes, the addition chlorination of benzene to benzene hexachloride yields a product which is primarily a mixture'of -benzene hexachloride isomers, the gamma isomer being only about to of the final chlorination product. Various methods have been devised to separate out components of the product which are particularly high in gamma content, which high gamma concentration materials are then used in the compounding of insecticide formulations. The remaining isomers of benzene hexachloride, however, have been of little useto date and, in many instances, create a disposal problem.

The conversion of these presently undesirable benzene hexachloride isomers into trichlorobenzene, which has considerable commercial utility, through dehydrohalog'e'nation of the benzene hexachloride in thepresence of activated carbon as a catalyst is, therefore, an important part of our present invention.

The preparation of t'richlorobenzenefrom benzene hex- -achloride by pyrolysis without .a catalyst 'or by decomposition through the use of alkalishas certain'objection- :able features. When pyrolysis without the aid of a catalyst is employed, the high temperatures required to decompose the benzene hexachlor ide to trichlorobenzene tend to produce a substantial amount of a black gummy residue. This gummy material must be removed in order to obtain trichlorobenzene in a satisfactorily pure state. When the benzene hexachloride is decomposed to the trichlorobenzene by the use of alkalis, such as refluxing the benzene hexachloride with sodium or potassiumhydroxide in water or alcohol, the conversion obtained is relatively :small. Also, the beta isomer is substantially unaffected. During the alkali method, some phenol compounds are :formed and substantial amounts of salts are produced, ;vhich materials inthemselves constitute a disposal probem.

By the production of trichlorobenzene from benzene Thexachloride through the use of activated carbon as a catalyst in accordance Withthe present invention, the time for producing the trichlorobenzene is considerably re-' duced and the problems adherent to straight pyrolytic decomposition and the method employing alkalis are avoided. Also, by carrying out :the reaction in the presence of :activatedcarbon substantially pure :tri chlorobenzene is obtained, the only by-product being HCl which is obtained in a sufficiently pure state to enable its packaging and sale Without further purification except possibly for the removal of any entrained organics. This is easily accomplished by passing the gas through a column packed with activated carbon.

The presently used commercial process for preparing trichlorobenzene is to chlorinate benzene in an iron reaction kettle in the absence iof lig'ht. Ferric chloride formed during the chlorination process acts as a catalyst to aid in the substitution chlorination of the benzene. With this process the maximum yield of trichlorobenzene obtainable in any given run is approximately 57% calculated on the basis of the benzene employed. The remaining products are primarily both under and over chlorinated benzenes. These must be separated from the desired trichlorobenzene product together with the ferric chloride formed during the chlorination reaction. This separation of the trichlorobenzene from the side reaction product entails the use of expensive equipment as well as considerable time for handling.

When preparing trichlorobenzene in accordance with our present invention, these objectionable aspects of the prior art process are eliminated.' The heretofore waste benzene hexachloride is converted directly to trichlorobenzene of sufliciently pure grade to be marketable as such without any further treatment other than possible scrubbing'with alkali to remove any entrained HCl. The only side product is HCl which is in substantially pure form. The yields of commercial grade trichlorobenzene obtained by our process are also considerably improved, yields as high as 99.5% of the theoretical having been obtained.

When preparing trichlorobenzene in accordance with our present invention, the benzene hexachloride is placed in a suitable container in contact with the activated carbon and heated to a temperature of approximately to 300 C. Temperatures higher than 300 C. can, of course, be employed if desired; the activated carbon would still aid in speeding up the reaction. However, high temperatures are objectionable for the reasons heretofore given. The reaction may, if desired, be carried out in a liquid medium, for example, by dissolving the benzene hexachloride in liquid trichlorobenzene, .or the material may be introduced into the presence of the activated carbon in the form of solid or molten benzene hexachloride. In either case, the activated carbon acts to considerably reduce the temperature required for the dehydrohalogenation reaction. When carried out in a liquid medium, the dehydrochlorination is preferably carried out at a temperature of 200 to 230 C;

The HCl resulting from the 'dehydrohalogenation of the benzene hexachloride is continuously Withdrawn from the reaction vessel during the process and is preferably collected in water, where it is absorbed. One of the ad vantages of our process is that the HCl evolved from the reaction is free from chlorine and of sufiicient purity that, after possible treatment to remove entrained organics, it can be sold, chlorine free, or used directly as a high grade muriatic acid. Also, the trichlorobenzene product is substantially pure, no phenolic materials-having been noted; there is, therefore, no problem with respect to disposal of undes'irable side reaction products.

Our invention is further illustrated by the following examples: V

EXAMPLE 1 i i Aone liter, three-necked reaction flask was charged to approximately one half its capacity with activated carbon pellets. Trichlorobenzene was then added in an amount sufiicient to cover the carbon. The flask was fitted with a thermometer, an addition port for solids and a two foot distillation column. The trichlorobenzene was heated to reflux temperatures (190 to 220 C.) and a portion of benzene hexachloride solids was added. There was an immediate evolution of "HCl gas and trichlorobenzene b g n tto distill from the top of the distillation column;

Addition of the benzene hexachloride was continued until 1,191 grams had been added. When the liquid in the flask had reached the same volume as the initial charge of trichlorobenzene, the distillation was discontinued. At the end of this time 740 grams of trichlorobenzene product had been collected, this amount represented on a Weight basis 99.5% of the theoretical yield based on the benzene hexachloride used. The trichlorobenzene initially present in the reaction flask acted both as a solubilizing agent and a heat transfer medium.

EXAMPLE 2 5 grams of hexachlorothiolane was heated in a small flask equipped with a reflux condenser to 230 C. at which temperature refluxing occurred. A small amount of HCl was liberated indicating slight decomposition of the hexachlorothiolane. 1 gram of Columbia Activated Carbon SXW of 6-8 mesh was added. Immediately there was a rapid evolution of HCl and the vapor temperature dropped to 175 C. This temperature was maintained for a few minutes, but as the amount of HCl evolved decreased the temperature slowly rose. After two hours the HCl evolved was nearly negligible and the temperature had risen to 228 C. Further addition of activated carbon caused no further release of HCl indicating that the reaction had gone to substantial completion. The product on analysis was found to be tetrachlorothiophene.

EXAMPLE 3 10 grams of tetrachlorothiolane was heated in a small distillation flask to 180 C. There was at this temperature a very slow evolution of HCl from the molten mass. On addition of 1 gram of Columbia Activated Carbon SXW, HCl was given off rapidly. Heating was continued at reflux temperature until no further evolution of HCl was apparent. The resulting product on distillation from the flask was found, after analysis, to be substantially pure dichlorothiophene.

EXAMPLE 4 EXAMPLE 5 10 grams of heptachlorocyclohexane was heated to approximately 240 C. and 1 gram of Columbia Activated Carbon SXW was added to the molten mass. HCl gas was given off rapidly, the rapid evolution of HCl beginning as soon as the activated carbon was added. The product on analysis was found to be tetrachlorobenzene.

EXAMPLE 6 8 grams of octachlorocyclohexane was heated in the presence of 1 gram of Columbia Activated Carbon SXW as described in Example 5. Again HCl was given 01f rapidly after the addition of activated carbon. The final product was a solid having a melting point of 8385 C. and found to be, on analysis, pentachlorobenzene.

In describing our invention, examples have been given for the preparation of a few specific organic compounds of aromatic character, trichlorobenzene, hexachlorobenzene, tetrachlorothiophene and dichlorothiophene. Our discovery and invention, however, is not limited to the preparation of these specific compounds. Neither is our invention limited to the specific activated carbons mentioned, since the invention in its broader aspects consists in the discovery that activated carbons as a class will catalyze reactions of the type wherein halogenated cyclic compounds by pyrolytic dehydrohalogenation are converted to compounds having conjugated systems and possessing aromaticity.

Having thus described our invention, we claim:

1. The method of making a chlorobenzene containing at least 4 chlorine atoms comprising heating a chlorocyclohexane containing at least 7 chlorine atoms to a temperature of at least C. in the presence of an activated carbon catalyst.

2. The method of making hexachlorobenzene comprising heating nonochlorocyclohexane to a temperature of at least 190 C. in the presence of an activated carbon catalyst.

3. The method of making pentachlorobenzene comprising heating octachlorocyclohexane to a temperature of at least 190 C. in the presence of an activated carbon catalyst.

4. The method of making tetrachlorobenzene comprising heating heptachlorocyclohexane to a temperature of at least 190 C. in the presence of an activated carbon catalyst.

No references cited. 

1. THE METHOD OF MAKING A CHLOROBENZENE CONTAINING AT LEAST 4 CHLORINE ATOMS COMPRISING HEATING A CHLOROCYCLOHEXANE CONTAINING AT LEAST 7 CHLORINE ATOMS TO A TEMPERATURE OF AT LEAST 190*C. IN THE PRESENCE OF A ACTIVATED CARBON CATALYST. 